Metals such as stainless steel as well as nickel, cobalt, titanium and tungsten based superalloys are frequently coated with a corrosion resistant material. One such corrosion resistant coating is a metalide coating, in particular, nickel aluminide coating. One method of applying such a metalide coating is disclosed in U.S. Pat. No. 5,334,417. Platinum and MCrAlY wherein the M represents a nickel cobalt alloy also form corrosion resistant surfaces. These metals cannot be applied as coatings using braze alloys. The melt suppressants in the braze alloy promote oxidation and corrosion and therefore are unsuitable for this application. As such, these coatings are typically applied using a plasma spray. The plasma spray apparatus is expensive and not particularly suitable for small or localized repairs.
Accordingly, it is an object of the present invention to provide a method to form a platinum or MCrAlY coating onto a superalloy surface without the use of a plasma spray.
Further, it is an object of the present invention to use a metalide coating to bond platinum or MCrAlY to the surface of a superalloy. The platinum or MCrAlY coating is formed on the surface of the metal part by coating the surface of the metal part with particles of platinum or MCrAlY and subsequently forming a metalide coating on the surface. Preferably the MCrAlY or platinum particles are held on the surface of the metal part using a binder such as PTFE or acrylic. The metalide coating is preferably applied by first forming a tape which includes metal such as aluminum, a halide carrier, metal oxide and a binder. The tape is placed over the coating of the corrosion resistant metal particles and the part being coated is then heated to cause the aluminum to react with the halide to form a metal halide compound which in turn will react with the metal surface, forming an aluminide coating. The aluminide coating bonds the corrosion resistant metal particles to the surface of the part being coated.
In an alternate embodiment of the present invention the corrosion resistant metal particles are simply blended with a binder such as polytetrafluoroethylene and placed onto the surface of the part being coated and a metalide tape is then placed over the corrosion resistant metal particle tape. The part is then subjected to a heating cycle to form the metalide coating to bond the corrosion resistant particles to the surface of the part.
In another alternate embodiment of the present invention the corrosion resistant particles are suspended in a liquid binder or adhesive and applied to the side of the aluminide coating tape to be placed against the part being coated.
In a further alternate embodiment of the present invention, a single layer coating tape includes platinum aluminum alloy in combination with optionally metal such as aluminum, the halide carrier, metal/oxide and binder. This tape is applied directly to the surface of the metal part being coated and is again subjected to a heat cycle which causes the platinum aluminum alloy to react with the halide forming the platinum aluminum halide complex. This in turn reacts the surface of the metal being coated, forming a platinum aluminide coating which is corrosion resistant.
The objects and advantages of the present invention will be further appreciated in light of the following detailed descriptions and drawings in which: